Using oxidation states to predict the outcome of reactions

The outcome of the reaction of transition-metal containing ions with various reagents can be predicted provided the properties of the reagent are known.   Various classifications for reagents are given below.
  • Reagents that are reductants donates electron to the transition metal-containing species, and the product arising contains the transition metal in a lower oxidation state.

    Reductants take you down!
    Common reductants are Sn2+, HSO3–, I–- and metals 
     
  • Reagents that are oxidants accept electrons from the transition metal-containing species, and the product arising contains the transition metal in a higher oxidation state.

    Oxidants take you up!
    Common oxidants are O2 and the halogens (Cl2, Br2 and I2).

     
  • Provided the transition metal is in a positive oxidation state, reaction with water, aqueous acid or alkali results in a change in speciation, not a change in oxidation number.
    M(OH)n may dissolve in acid (H+).
    The metal-containing product in acid will be a cation or oxocation (Mn+ for example). 

    M(OH)n may dissolve in base (HO–).
    The metal-containing product in base will be an hydroxo or oxoanion (M(OH)nm– for example)

    Reaction with water gives rise to uncharged products containing either HO– or O2–  as the anion.
Dr Sheila Woodgate
© The University of Auckland 2002